Laser treatment device, laser treatment method, and semiconductor device fabrication method

ABSTRACT

Object of the Invention 
     To obtain thin film transistors with controlled characteristics on a substrate. 
     Means 
     A semiconductor film formed on a substrate is crystallized by continuously oscillating type laser. The scanning direction of the continuously oscillating type laser and the crystallization direction are coincident with each other. Adjustment of the crystallization direction and the charge transferring direction of the thin film transistors makes control of the characteristics of the thin film transistors possible. With respect to the laser treatment device for crystallizing the semiconductor film, the beam shape of laser oscillated from the continuously oscillating type laser device is made to be elliptical by a cylindrical lens and said cylindrical lens is made rotatable and said laser beam is scanned on said substrate by a galvanomirror and said laser beam can be focused upon said substrate by f-θ lens.

DETAILED DESCRIPTION OF THE INVENTION

1. Technical Field to which the Invention Belongs

The present invention relates to a method for annealing a semiconductorfilm using a laser beam (hereinafter referred to as laser annealing) anda laser irradiation device for performing the laser annealing (devicesincluding a laser and an optical system for guiding a laser beam outputfrom the laser to a member to be processed). Further, the presentinvention relates to a semiconductor device fabricated by the stepsincluding the laser annealing step and a method for manufacturing thesemiconductor device. Note that the semiconductor device mentionedthroughout the specification includes an electro-optical device such asa liquid crystal display device and an EL display device.

2. Prior Art

In recent years, development of thin film transistors (hereinbelowreferred to as TFTs) has been advanced, and in particular, TFTsemploying a polycrystalline silicon film (polysilicon film) as acrystalline semiconductor film has drawn much attention. Especially, ina liquid crystal display device (liquid crystal display) or an EL(electro-luminescence) display device (EL display), these TFTs are usedas devices for switching pixels and devices constituting a drivercircuit to control the pixels.

A polycrystalline semiconductor film is used for the activation layer ofthe TFTs. By adding impurities, a source region, drain region andchannel are formed in the polycrystalline semiconductor film. Inaddition, OFFSET regions and LDD regions may be provided.

In common techniques for obtaining a polysilicon film, an amorphoussilicon film is crystallized to obtain a polysilicon film. Inparticular, a method of crystallizing an amorphous silicon film withlaser beams has been receiving much attention. In the presentspecification, the technique for crystallizing an amorphoussemiconductor film with laser beams to obtain a crystallinesemiconductor film is referred to as laser crystallization.

The laser crystallization enables instantaneous heating of asemiconductor film, and thus it is an effective technique for annealinga semiconductor film formed on a substrate having low heat-resistance,such as a glass substrate, a plastic substrate or the like.

Various kinds of laser beams are available, which can be roughly dividedinto continuous oscillation type and pulse oscillation type. In respectto the laser crystallization, excellent crystalline properties can beobtained by using the continuous oscillation laser beam, thereby thecontinuous oscillation laser beam has drawn attention.

Problems to be Solved by the Invention

With respect to thin film transistors formed on one sheet of asubstrate, the invention aims to produce thin film transistorssatisfying required characteristics all while individually producingthem by an innovative optical system of a continuously oscillating typelaser.

In a liquid crystal display device and an EL display device, devices forswitching pixels and devices composing driving circuits for controllingthe pixels are formed in one substrate. These devices are required tohave a variety of characteristics corresponding to their roles. However,it is difficult for thin film transistors to satisfy a variety ofcharacteristics required in the case they are produced by a method forobtaining crystalline semiconductor films by evenly radiatingcontinuously oscillating laser to an amorphous silicon film formedentirely on a substrate.

Means for Solving the Problems

Thin film transistors and crystallinity of semiconductor films will bedescribed. Electric properties of thin film transistors greatly dependon the crystallinity of semiconductor films. Specially, the boundariessuch as grain boundaries between crystals and other crystals hindercarrier movement. Owing to the hindrance of the movement of carriers,the electric resistance of thin film transistors increases. Accordingly,in order to control the electric properties of thin film transistors, itis required to control the number of the grain boundaries.

The number of grain boundaries can be controlled by the crystal growthdirection. The crystal growth direction can not be controlled bycarrying out crystallization by heating a substrate, whereas the crystalgrowth direction can be controlled by laser crystallization because itcan carry out local heating and melting.

The growth direction of a crystal of a semiconductor film can becontrolled by making the beam shape of laser beam oscillated from acontinuously oscillating type laser linear and making the scanningdirection coincide with the width direction of the beam. In this case,as shown in FIG. 2, the growth direction of the crystal of thesemiconductor film follows the scanning direction of laser beam. Thecrystal growth direction can be understood from the grain boundaryformation direction. Here, it becomes important that the beam is madelinear and scanned on an object to be radiated (practically asubstrate). Further, the beam shape of laser beam is precisely made tobe elliptical or rectangular, however the aspect ratio is high andtherefore, here, it is defined as linear. An optical system constitutingthe invention is illustrated in FIG. 1 a and hereinafter, the systemwill be described.

Oscillated circular laser beam is diffused uniaxially by a cylindricallens 102. Adjustment of the focal distance by a f-θ lens 104 is madepossible to focus (to adjust the focal point of) the laser beam on anyposition of a substrate. The phrase, to focus (to adjust the focal pointof), means to make the linear beam shape and the size on an object to beradiated even on any point of the object to be radiated. The incidentlight to the f-θ lens 104 can be controlled by a galvanomirror 103. Thatis, by moving the galvanomirror 103, the radiation position of laserbeam can be changed to scan on the substrate. In such a manner, withoutmoving the substrate, laser beam with an elliptical shape (a linearshape) can be scanned on the substrate.

If the energy density of laser beam per unit surface area is required tochange, it may be supposedly possible to insert a lens into this opticalsystem, to change the distance of the substrate, and the like. Bychanging the focal distance of the cylindrical lens 102, the beam shapecan be adjusted. Further, the scanning speed can be changed by thegalvanomirror 103.

Here, what is important is to rotate the cylindrical lens 102 so as torotate the linear beam projected to the substrate. Generally, a largenumber of thin film transistors of active matrix circuits or the likeformed on a substrate exist and their orientation, that is, the chargetransferring direction, is varied. Accordingly, just like the invention,electric characteristics of the respective thin film transistors on thesubstrate can be controlled by making the cylindrical lens 102 rotatableand scanning the laser beam in the linear direction (the widthdirection) in combination to control the crystal growth direction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a laser radiation device and a treatment substrate of theinvention.

FIG. 2 shows a semiconductor film crystallized by laser beam withelliptical shape.

FIG. 3 shows thin film transistors with controlled crystal growthdirection.

FIG. 4 shows a beam expander in the Embodiment 3.

FIG. 5 shows a laser radiation device of the invention in the Embodiment8.

FIG. 6 shows a laser radiation device of the invention in the Embodiment8.

FIG. 7 shows an EL display device bearing peripheral driving circuitsand pixel circuits of Example 1.

FIG. 8 shows a magnified figure of the pixel circuits of Example 1.

FIG. 9 shows a figure showing the fabrication process of TFT of an ELdisplay device.

FIG. 10 shows a figure showing the fabrication process of TFT of an ELdisplay device.

FIG. 11 shows a figure showing the fabrication process of TFT of an ELdisplay device.

FIG. 12 shows a laser radiation device and a treatment substrate of theinvention.

PREFERRED EMBODIMENT OF THE INVENTION Embodiment 1

One embodiment of the invention will be described. An embodiment of theinvention is, with respect to thin film transistors having chargetransferring directions in the vertical direction and the transversedirection of the substrate, to fabricate thin film transistors withcrystal growth direction coincident with the charge transferringdirection.

FIG. 1 a shows the constitution of a laser device comprising laser ofthe invention. It comprises a continuously oscillating type Nd:YVO₄laser 101, a convex cylindrical lens 102 for processing laser beam(second harmonic; wavelength of 532 nm) oscillated from the continuouslyoscillating Nd:YVO₄ laser 101 to be elliptical, a galvanomirror 103 forscanning the laser beam, a f-θ lens 104 for adjusting the focaldistance, and a stage 105 for fixing a substrate. The convex cylindricallens 102 is installed on a rotation stage so as to be rotatedoptionally. An amorphous silicon film is formed on the substrate and aregion A and a region B are formed. FIG. 1 b is a top view of thesubstrate showing there are thin film transistors which have a chargetransferring direction in the transverse direction of the substrate andare to be formed in the region A and thin film transistors which have acharge transferring direction in the vertical direction of the substrateand are to be formed in the region B.

In this case, the output power of laser is 10 W. The laser beam shape isan ellipse with a length of 20 μm in the minor axis direction and alength of 400 μm in the major axis direction. The scanning speed is 20cm/s. Melting of a semiconductor film depends on the energy density perunit surface area. Accordingly, if energy density is similar to thatdescribed above, the shape or the size of the laser beam may be changed.

With such a constitution, laser beam is radiated to an amorphous siliconfilm. In the region A, the power direction of the convex cylindricallens 102 is set to be in (a) direction by a rotation stage (details notillustrated) and laser beam is scanned in the transverse direction ofthe substrate by the galvanomirror 103. On the other hand, in the regionB, the power direction of the convex cylindrical lens 102 is set to bein (b) direction at an angle of 90° turned from the (a) direction by therotation stage and laser beam is scanned in the vertical direction ofthe substrate.

By doing that, thin film transistors with the even laser beam scanningdirection, that is, the crystal growth direction parallel to the chargetransferring direction, can be fabricated. The thin film transistorsfabricated by such a manner have crystal grain boundaries shown in FIG.3 a. Incidentally, the crystal grain is a single crystal. Depending onthe electric characteristic which the thin film transistors are requiredto have, the laser beam scanning direction, that is, the crystal growthdirection may be perpendicular to the charge transferring direction. Inthe case of making the directions perpendicular, since a plurality ofgrain boundaries exist in the charge transferring direction of the thinfilm transistors as shown in FIG. 3 b, the mobility of carriers isdecreased. However, there is an advantage that the leak current can bedecreased in off-state of switches of the thin film transistors.

Further, the laser beam is scanned by the galvanomirror 103 and thelaser beam shape can be controlled by the convex cylindrical lens 102,so that crystallization can be carried out without moving the substrate.

Embodiment 2

Another embodiment different from the Embodiment 1 will be explained.This embodiment is, with respect to thin film transistors having chargetransferring directions in the vertical direction and the transversedirection of the substrate, to fabricate thin film transistors withcrystal growth direction different from the charge transferringdirection and uniform electric characteristics. An optical system and asubstrate with the same constitutions as those shown in FIGS. 1 a and 1b are employed.

The convex cylindrical lens 102 installed in the rotation stage is setat 45° between (a) and (b). When laser beam is oscillated with such aconstitution, elliptical shape is projected in the diagonal direction tothe substrate. When the laser beam is scanned in the minor axisdirection, the crystal growth takes place in the direction at 45° fromthe vertical direction or the transverse direction.

From the substrate crystallized in such a manner, thin film transistorsas shown in FIG. 3 c are to be fabricated. The thin film transistorshaving the charge transferring direction in the vertical direction ofthe substrate and the thin film transistors having the chargetransferring direction in the transverse direction of the substrate areto have approximately the same number of grain boundaries crossing thecharge transferring direction, so that the electric characteristicsbecome uniform.

As shown in FIG. 12, in the case where thin film transistors are to beformed alternatively in vertical and transverse directions to make thecharacteristics united, the method also can be applied.

Further, depending on the size and shape of the thin film transistors,the angle of the cylindrical lens is not necessarily set at 45°. Sinceit is only required to make electric characteristics of the thin filmtransistors uniform, the cylindrical lens may be rotated between 0° and90° and scanning is carried out in the minor axis direction. The crystalgrowth direction of the semiconductor film to be employed for the thinfilm transistors formed in such a manner are not parallel to thevertical direction and the transverse direction but a diagonaldirection.

Embodiment 3

In this embodiment, an optical system with which the optical adjustmentis made easy to carry out at the time of making the beam shapeelliptical will be described. In the Embodiments 1 and 2, the convexcylindrical lens for forming the elliptical shape is one in number. Onthe other hand, in this embodiment, two lenses are employed.

There is a Galileo way and a Kepler way as the methods for magnifying orreducing a beam by two lenses. The Galileo way is a method for changingthe beam size using a concave lens and a convex lens as shown in FIG. 4.On the other hand, the Kepler way is a method for changing the beam sizeusing two convex lenses as shown in FIG. 4.

If the lenses are changed to cylindrical lenses, the beam can bemagnified or reduced only in one axis direction and the ratio of themajor axis and the minor axis can be easily adjusted. Nevertheless, thetwo cylindrical lenses are required to be even in the axis direction.Accordingly, they have to be moved simultaneously at the time ofrotating them on the rotation stage or the like.

Other than cylindrical lenses, two pairs of prisms may be used formagnification or reduction only in one axis direction. Besides these,those having equivalent function may be also employed.

Embodiment 4

In this embodiment, an optical system for making the length in the minoraxis direction further shorter will be described. The width of laserbeam at the focal point converged by a lens is determined on the basisof the focal distance of the lens and the spread angle of the laser.Laser beam has wavefront in Laplace-Gauss distribution and because ofthat, the beam diameter becomes wide at the focal point. Accordingly, inthis embodiment, a computer hologram for making parallel the wavefrontin Laplace-Gauss distribution is introduced into the optical system. Ifthe wavefront is parallel, the beam width at the focal point can benarrowed in Laplace-Gauss distribution.

The computer hologram is effective to be inserted before the lens.Accordingly, in the case shown in FIG. 1 a, it is inserted before thecylindrical lens 102. In the case that radiation with the same laserpower is carried out, when making the length in the minor axis directionshorter, the length of the elliptical shape in the major axis directioncan be extended. Consequently, the number of scanning times per unitsurface area can be decreased to result in a speedup of the substratetreatment.

Embodiment 5

In this embodiment, a device actually required for carrying outradiation to a substrate will be described. As shown in FIG. 1 a, in thecase beam is scanned on the substrate by a galvanomirror, the beamradiated in the substrate center is reflected and turned back to thelaser device. This return laser beam causes damages on harmonicconversion devices. Accordingly, in order to avoid that, a device suchas an isolator, which allows only beam in one direction to pass, isintroduced into the optical system.

In the FIG. 1 a, it is effective to be inserted before the cylindricallens where the laser beam shape is not changed. The foregoing computerhologram may be employed in combination in optional arrangement order.

Embodiment 6

In this embodiment, another device different from that for theEmbodiment 5 and required for actually carrying out radiation to thesubstrate will be described. The laser beam to be scanned on a substratemelts the substrate if it is radiated to the substrate for a longduration because of the high energy density. Accordingly, in the casethat the laser beam scanning is stopped due to a cause such as a troubleof a galvanomirror or the like, the radiation to the substrate has to bestopped. Therefore, in this invention, an interlock is installed infront of the galvanomirror. The interlock starts operation and shuts thebeam by a metal plate or the like when abnormality is detected or atrouble of the galvanomirror is detected by a sensor installed on astage.

Embodiment 7

In this embodiment, another device different from those of Embodiments 5and 6 and required for actually carrying out radiation to the substratewill be described. Since the laser beam to be scanned on a substrate isextremely intense, even the reflected light and transmitted light whichare not absorbed in an amorphous silicon film cause damage in thecircumference. Accordingly, in order to prevent diffusion of such light,a chamber or a damper is required to be installed in the surrounding ofthe substrate.

Embodiment 8

In this embodiment, an optical system required for mass productionsystem will be described. As shown in FIG. 5, it is for simultaneouslyscanning of laser beam from two light sources by controlling agalvanomirror 503 with one controller. By doing that, the throughput ofthe laser crystallization can be increased by two times.

As another method for simultaneous scanning of laser beam from twosources, as shown in FIG. 6, laser beam is radiated from the front sideof a substrate and another kind of laser beam is radiated from the rearside of the substrate. However, the substrate and the stage are requiredto transmit laser beam.

The invention includes combinations of any of the Embodiments 1 to 8.Further, although laser beam is radiated to the front side of theamorphous silicon film in the Embodiments 1 to 8, if the laser beam haswavelength proper to be transmitted through the substrate and absorbedin semiconductor films, it can be radiated from the rear side of thesubstrate.

EXAMPLES Example 1

Described with reference to FIGS. 9 to 11 is an example of the presentinvention. This example describes a method for manufacturing TFTs havingits charge transferring direction in the vertical direction and thetransverse direction of a substrate. FIG. 7 is a top view of an ELdisplay device in which a peripheral driving circuit and a pixel circuitare formed on one substrate. The TFTs having its charge transferringdirection in the vertical direction of the substrate and the TFTs havingits charge transferring direction in the transverse direction of thesubstrate are mixed to constitute a peripheral driving circuit. Theabove view of FIG. 7 illustrates the TFTs constituting an invertercircuit, which is a part of the peripheral driving circuit, and the TFTshaving its charge transferring direction in the vertical direction areformed. On the other hand, the pixel circuit has a constitution whereplural pixels are formed in matrix, a state shown in FIG. 8 can beobtained by enlarging one part thereof. The pixel thereof is constitutedonly by the TFTs having its charge transferring direction in thetransverse direction.

Here, a detailed description of manufacturing switching TFTs and drivingTFTs that constitute the pixel circuit and the TFTs constituting theinverter circuit of the peripheral circuit at the same time will beshown in accordance with the process. Further, since the erasing TFTscan be fabricated simultaneously with the switching TFTs, therefore, thedescription is omitted.

In FIG. 9, for the substrate 900, other than a glass substrate made ofglass such as barium borosilicate glass or aluminoborosilicate glass asrepresented by the glass #7059 or the glass #1737 of Corning Co., aquartz substrate, a plastic substrate using polyethyleneterephthalate,polyethylenenaphthalate, polyethersulfone and the like may be used.

In order to prevent impurities diffusion from the substrate 900, on thesurface of the substrate 900 on which the TFT is to be formed, anunderlying film 901 comprising an insulating film such as silicon oxidefilm, silicon nitride film or silicon oxynitride film is formed. In thisexample, the underlying film 901 has a two-layer structure. There,however, may be employed a structure in which a single layer or two ormore layers are laminated on the insulating film. The first layer of theunderlying film 901 is a silicon oxynitride film 901 a formedmaintaining a thickness of from 10 to 200 nm (preferably, from 50 to 100nm) relying upon a plasma CVD method by using SiH₄, NH₃ and N₂O asreaction gases. In this example, the silicon oxynitride film 901 a(having a composition ratio of Si=32%, O=27%, N=24%, H=17%) is formedmaintaining a thickness of 50 nm. The second layer of the underlyingfilm 901 is a silicon oxynitride film 901 b formed maintaining athickness of from 50 to 200 nm (preferably, from 100 to 150 nm) relyingupon the plasma CVD method by using SiH₄ and N₂O as reaction gases. Inthis example, the silicon oxynitride film 901 b (having a compositionratio of Si=32%, O=27%, N=24%, H=17%) is formed maintaining a thicknessof 100 nm.

Then, an amorphous semiconductor film 903 is formed maintaining athickness of 25 to 150 nm (preferably 30 to 60 nm) by a known methodsuch as a plasma CVD method, LPCVD method, sputtering method and thelike. There is no limitation on the material of the semiconductor film,however, there is preferably used silicon or a silicon-germanium(Si_(x)Ge_(1-x) (X=0.0001 to 0.02)) alloy thereto. In this example, theamorphous silicon film is formed maintaining a thickness of 55 nmrelying on the plasma CVD method.

The amorphous silicon film on the substrate is irradiated from the abovedirection by a laser irradiation device as shown in FIG. 1A. Region A isthe pixel circuit, as shown in FIG. 8, where the TFTs having its chargetransferring direction in the transverse direction are formed. Here,cylindrical lens are set to be in “a” direction, and the laser beam isirradiated in the transverse direction of the substrate by agalvanomirror.

Here, in the pixel circuit, although thin film transistors with the evenlaser scanning direction, that is, the crystal growth direction parallelto the charge transferring direction, is fabricated, depending on theelectric characteristics which the thin film transistors are required tohave, the laser beam scanning direction perpendicular, that is, thecrystal growth direction perpendicular to the charge transferringdirection. In the case of making the directions perpendicular, since aplurality of grain boundaries exist in the charge transferring directionof the thin film transistors as shown in FIG. 3 b, the mobility ofcarriers is decreased. However, there is an advantage that the leakcurrent can be decreased in off-state of switches of the thin filmtransistors.

On the hand, Region B is the peripheral driving circuit, which is formedby mixing the TFTs having its charge transferring direction in thevertical direction of the substrate and the TFTs having its chargetransferring direction in the transverse direction of the substrate.Here, the cylindrical lens is set at 45° between (a) and (b), the laserbeam is scanned in the diagonal direction of the substrate by thegalvanomirror, in the strict sense, in the minor axis direction of thebeam of an elliptical shape projected in the substrate.

Both the TFTs having its charge transferring direction in the verticaldirection of the substrate and the TFTs having its charge transferringdirection in the transverse direction of the substrate exist in theperipheral driving circuit. However, since identical frequency isrequired in the circuit, electric characteristics of the respective thinfilm transistors on the substrate need to be uniformed. Accordingly, asdescribed above, scanning is performed at 45° of the vertical directionor the transverse direction of the substrate. In addition, the angle ofthe cylindrical lens can be adjusted depending on the arrangement of thecircuit.

The crystalline silicon film is patterned by a photolithographic methodto form semiconductor layers 902 to 905.

The semiconductor layers 902 to 905 that have been formed may further bedoped with trace amounts of an impurity element (boron or phosphorus) tocontrol the threshold value of the TFT

Then, a gate insulating film 906 is formed to cover the semiconductorlayers 902 to 905. The gate insulating film 906 is formed of aninsulating film containing silicon maintaining a thickness of from 40 to150 nm by the plasma CVD method or the sputtering method. In thisexample, the gate insulating film is formed of a silicon oxynitride film(composition ratio of Si=32%, O=59%, N=7%, H=2%) maintaining a thicknessof 110 nm by the plasma CVD method. The gate insulating film is notlimited to the silicon oxynitride film but may have a structure on whichis laminated a single layer or plural layers of an insulating filmcontaining silicon.

When the silicon oxide film is to be formed, TEOS (tetraethylorthosilicate) and O₂ are mixed together by the plasma CVD method, andare reacted together under a reaction pressure of 40 Pa, at a substratetemperature of from 300 to 400° C., at a frequency of 13.56 MHz and adischarge electric power density of from 0.5 to 0.8 W/cm². The thusformed silicon oxide film is, then, heat annealed at 400 to 500° C.thereby to obtain the gate insulating film having good properties.

Then, a heat resistant conductive layer 907 is formed on the gateinsulating film 906 maintaining a thickness of from 200 to 400 nm(preferably, from 250 to 350 nm) to form the gate electrode. Theheat-resistant conductive layer 907 may be formed as a single layer ormay, as required, be formed in a structure of laminated layers of plurallayers such as two layers or three layers. The heat resistant conductivelayer contains an element selected from Ta, Ti and W, or contains analloy of the above element, or an alloy of a combination of the aboveelements. The heat-resistant conductive layer is formed by thesputtering method or the CVD method, and should contain impurities at adecreased concentration to decrease the resistance and should,particularly, contain oxygen at a concentration of not higher than 30ppm. In this example, the W film may be formed by the sputtering methodby using W as a target, or may be formed by the hot CVD method by usingtungsten hexafluoride (WF₆). In either case, it is necessary to decreasethe resistance so that it can be used as the gate electrode. It is,therefore, desired that the W film has a resistivity of not larger than20 μΩcm. The resistance of the W film can be decreased by coarsening thecrystalline particles. When W contains much impurity elements such asoxygen, the crystallization is impaired and the resistance increases.When the sputtering method is employed, therefore, a W target having apurity of 99.999% or 99.99% is used, and the W film is formed whilegiving a sufficient degree of attention so that the impurities will notbe infiltrated from the gaseous phase during the formation of the film,to realize the resistivity of from 9 to 20 μΩcm.

On the other hand, the Ta film that is used as the heat-resistantconductive layer 907 can similarly be formed by the sputtering method.The Ta film is formed by using Ar as a sputtering gas. Further, theaddition of suitable amounts of Xe and Kr into the gas during thesputtering makes it possible to relax the internal stress of the filmthat is formed and to prevent the film from being peeled off. The Tafilm of α-phase has a resistivity of about 20 μΩcm and can be used asthe gate electrode but the Ta film of β-phase has a resistivity of about180 μΩcm and is not suited for use as the gate electrode. The TaN filmhas a crystalline structure close to the α-phase. Therefore, if the TaNfilm is formed under the Ta film, there is easily formed the Ta film ofα-phase. Further, though not diagramed, formation of the silicon filmdoped with phosphorus (P) maintaining a thickness of about 2 to about 20nm under the heat resistant conductive layer 907 is effective infabricating the device. This helps improve the intimate adhesion of theconductive film formed thereon, prevent the oxidation, and prevent traceamounts of alkali metal elements contained in the heat resistantconductive layer 907 from being diffused into the gate insulating film906 of the first shape. In any way, it is desired that theheat-resistant conductive layer 907 has a resistivity over a range offrom 10 to 50 μΩcm.

Next, a mask 908 is formed by a resist relying upon thephotolithographic technology. Then, a first etching is executed. Thisexample uses an ICP etching device, uses Cl₂ and CF₄ as etching gases,and forms a plasma with RF (13.56 MHz) electric power of 3.2 W/cm² undera pressure of 1 Pa. The RF (13.56 MHz) electric power of 224 mW/cm² issupplied to the side of the substrate (sample stage), too, whereby asubstantially negative self bias voltage is applied. Under thiscondition, the W film is etched at a rate of about 100 nm/min. The firstetching treatment is effected by estimating the time by which the W filmis just etched relying upon this etching rate, and is conducted for aperiod of time which is 20% longer than the estimated etching time.

The conductive layers 909 to 913 having a first tapered shape are formedby the first etching treatment. The conductive layers 909 to 913 aretapered at an angle of from 15 to 30°. To execute the etching withoutleaving residue, over-etching is conducted by increasing the etchingtime by about 10 to 20%. The selection ratio of the silicon oxynitridefilm (gate insulating film 906) to the W film is 2 to 4 (typically, 3).Due to the over etching, therefore, the surface where the siliconoxynitride film is exposed is etched by about 20 to about 50 nm (FIG.9(B)).

Then, a first doping treatment is effected to add an impurity element ofa first type of electric conduction to the semiconductor layer. Here, astep is conducted to add an impurity element for imparting the n-type. Amask 908 forming the conductive layer of a first shape is left, and animpurity element is added by the ion-doping method to impart the n-typein a self-aligned manner with the conductive layers 909 to 913 having afirst tapered shape as masks. The dosage is set to be from 1×10¹³ to5×10¹⁴ atoms/cm² so that the impurity element for imparting the n-typereaches the underlying semiconductor layer penetrating through thetapered portion and the gate insulating film 906 at the ends of the gateelectrode, and the acceleration voltage is selected to be from 80 to 160keV. As the impurity element for imparting the n-type, there is used anelement belonging to the Group 15 and, typically, phosphorus (P) orarsenic (As). Phosphorus (P) is used, here. Due to the ion-dopingmethod, an impurity element for imparting the n-type is added to thefirst impurity regions 914 to 917 over a concentration range of from1×10²⁰ to 1×10²¹ atoms/cm³ (FIG. 9(C)).

In this step, the impurities turn down to the lower side of theconductive layers 909 to 912 of the first shape depending upon thedoping conditions, and it often happens that the first impurity regions914 to 917 are overlapped on the conductive layers 909 to 912 of thefirst shape.

Next, the second etching treatment is conducted as shown in FIG. 9(D).The etching treatment, too, is conducted by using the ICP etchingdevice, using a mixed gas of CF₄ and Cl₂ as an etching gas, using an RFelectric power of 3.2 W/cm² (13.56 MHz), a bias power of 45 mW/cm²(13.56 MHz) under a pressure of 1.0 Pa. Under this condition, there areformed the conductive layers 918 to 921 of a second shape. The endportions thereof are tapered, and the thickness gradually increases fromthe ends toward the inside. The rate of isotropic etching increases inproportion to a decrease in the bias electricity applied to the side ofthe substrate as compared to the first etching treatment, and the angleof the tapered portions becomes 30 to 60°. The mask 908 is etched at theedge by etching to form a mask 922. In the step of FIG. 9(D), thesurface of the gate insulating film 906 is etched by about 40 nm.

Then, the doping is effected with an impurity element for imparting then-type under the condition of an increased acceleration voltage bydecreasing the dosage to be smaller than that of the first dopingtreatment. For example, the acceleration voltage is set to be from 70 to120 keV, the dosage is set to be 1×10¹³/cm² thereby to form firstimpurity regions 924 to 927 having an increased impurity concentration,and second impurity regions 928 to 931 that are in contact with thefirst impurity regions 924 to 927. In this step, the impurity may turndown to the lower side of the conductive layers 918 to 921 of the secondshape, and the second impurity regions 928 to 931 may be overlapped onthe conductive layers 918 to 921 of the second shape. The impurityconcentration in the second impurity regions is from 1×10¹⁶ to 1×10¹⁸atoms/cm³ (FIG. 10(A)).

Referring to FIG. 10(B), impurity regions 933 (933 a, 933 b) and 934(934 a, 934 b) of the conduction type opposite to the one conductiontype are formed in the semiconductor layers 902, 905 that form thep-channel TFTs. In this case, too, an impurity element for imparting thep-type is added using the conductive layers 918 and 921 of the secondshape as masks to form impurity regions in a self-aligned manner. Atthis moment, the semiconductor layers 903 and 904 forming the n-channelTFTs are entirely covered for their surfaces by forming a mask 932 of aresist. Here, the impurity regions 933 and 934 are formed by theion-doping method by using diborane (B₂H₆). The impurity element forimparting the p-type is added to the impurity regions 933 and 934 at aconcentration of from 2×10²⁰ to 2×10²¹ atoms/cm³.

If closely considered, however, the impurity regions 933, 934 can bedivided into two regions containing an impurity element that imparts then-type. Third impurity regions 933 a and 934 a contain the impurityelement that imparts the p-type at a concentration of from 1×10²⁰ to1×10²¹ atoms/cm³ and fourth impurity regions 933 b and 934 b contain theimpurity element that imparts the n-type at a concentration of from1×10¹⁷ to 1×10²⁰ atoms/cm³. In the fourth impurity regions 933 b and 934b, however, the impurity element for imparting the p-type is containedat a concentration of not smaller than 1×10¹⁹ atoms/cm³ and in the thirdimpurity regions 933 a and 934 a, the impurity element for imparting thep-type is contained at a concentration which is 1.5 to 3 times as highas the concentration of the impurity element for imparting the n-type.Therefore, the third impurity regions work as source regions and drainregions of the p-channel TFTs without arousing any problem.

Referring next to FIG. 10(C), a first interlayer insulating film 937 isformed on the conductive layers 918 to 921 of the second shape and onthe gate insulating film 906. The first interlayer insulating film 937may be formed of a silicon oxide film, a silicon oxynitride film, asilicon nitride film, or a laminated layer film of a combinationthereof. In any case, the first interlayer insulating film 937 is formedof an inorganic insulating material. The first interlayer insulatingfilm 937 has a thickness of 100 to 200 nm. When the silicon oxide filmis used as the first interlayer insulating film 937, TEOS and O₂ aremixed together by the plasma CVD method, and are reacted together undera pressure of 40 Pa at a substrate temperature of 300 to 400° C. whiledischarging the electric power at a high frequency (13.56 MHz) and at apower density of 0.5 to 0.8 W/cm². When the silicon oxynitride film isused as the first interlayer insulating film 937, this siliconoxynitride film may be formed from SiH₄, N₂O and NH₃, or from SiH₄ andN₂O by the plasma CVD method. The conditions of formation in this caseare a reaction pressure of from 20 to 200 Pa, a substrate temperature offrom 300 to 400° C. and a high-frequency (60 MHz) power density of from0.1 to 1.0 W/cm². As the first interlayer insulating film 937, further,there may be used a hydrogenated silicon oxynitride film formed by usingSiH₄, N₂O and H₂. The silicon nitride film, too, can similarly be formedby using SiH₄ and NH₃ by the plasma CVD method.

Then, a step is conducted for activating the impurity elements thatimpart the n-type and the p-type added at their respectiveconcentrations. This step is conducted by thermal annealing method usingan annealing furnace. There can be further employed a laser annealingmethod or a rapid thermal annealing method (RTA method). The thermalannealing method is conducted in a nitrogen atmosphere containing oxygenat a concentration of not higher than 1 ppm and, preferably, not higherthan 0.1 ppm at from 400 to 700° C. and, typically, at from 500 to 600°C. In this example, the heat treatment is conducted at 550° C. for 4hours. When a plastic substrate having a low heat resistance temperatureis used as the substrate 900, it is desired to employ the laserannealing method.

Following the step of activation, the atmospheric gas is changed, andthe heat treatment is conducted in an atmosphere containing 3 to 100% ofhydrogen at from 300 to 450° C. for from 1 to 12 hours to hydrogenatethe semiconductor layer. This step is to terminate the dangling bonds of10¹⁶ to 10¹⁸/cm³ in the semiconductor layer with hydrogen that isthermally excited. As another means of hydrogenation, the plasmahydrogenation may be executed (using hydrogen excited with plasma). Inany way, it is desired that the defect density in the semiconductorlayers 902 to 905 be suppressed to be not larger than 10¹⁶/cm³. For thispurpose, hydrogen may be added in an amount of from 0.01 to 0.1 atomic%.

Then, a second interlayer insulating film 939 of an organic insulatingmaterial is formed maintaining an average thickness of from 1.0 to 2.0μm. As the organic resin material, there can be used polyimide, acrylic,polyamide, polyimideamide, or BCB (benzocyclobutene). When there isused, for example, a polyimide of the type that is heat polymerizedafter being applied onto the substrate, the second interlayer insulatingfilm is formed being fired in a clean oven at 300° C. When there is usedan acrylic, there is used the one of the two-can type. Namely, the mainmaterial and a curing agent are mixed together, applied onto the wholesurface of the substrate by using a spinner, pre-heated by using a hotplate at 80° C. for 60 seconds, and are fired at 250° C. for 60 minutesin a clean oven to form the second interlayer insulating film.

Thus, the second interlayer insulating film 939 is formed by using anorganic insulating material featuring good and flattened surface.Further, the organic resin material, in general, has a small dielectricconstant and lowers the parasitic capacitance. The organic resinmaterial, however, is hygroscopic and is not suited as a protectionfilm. It is, therefore, desired that the second interlayer insulatingfilm is used in combination with the silicon oxide film, siliconoxynitride film or silicon nitride film formed as the first interlayerinsulating film 937.

Thereafter, the resist mask of a predetermined pattern is formed, andcontact holes are formed in the semiconductor layers to reach theimpurity regions serving as source regions or drain regions. The contactholes are formed by dry etching. In this case, a mixed gas of CF₄, O₂and He is used as the etching gas, and the second interlayer insulatingfilm 939 of the organic resin material is etched. Thereafter, CF₄ and O₂are used as the etching gas to etch the first interlayer insulating film937. In order to further enhance the selection ratio relative to thesemiconductor layer, CHF₃ is used as the etching gas to etch the gateinsulating film 570 of the third shape, thereby to form the contactholes.

Here, the conductive metal film is formed by sputtering and vacuumvaporization and is patterned by using a mask and is, then, etched toform source wirings 940 to 943 and drain wirings 944 to 946. Further,though not diagramed in this example, the wiring is formed by a laminateof a 50 nm thick Ti film and a 500 nm thick alloy film (alloy film of Aland Ti).

Then, a transparent conductive film is formed thereon maintaining athickness of 80 to 120 nm, and is patterned to form a pixel electrode947 (FIG. 11 (A)). Therefore, the pixel electrode 947 is formed by usingan indium oxide-tin (ITO) film as a transparent electrode or atransparent conductive film obtained by mixing 2 to 20% of a zinc oxide(ZnO) into indium oxide.

Further, the pixel electrode 947 is formed being in contact with, andoverlapped on, the connecting wiring 946 that is electrically connectedto the drain region of driving TFTs 963.

Next, a third interlayer insulating film 949 having opening at aposition that coincides with the pixel electrode 947 is formed as shownin FIG. 11B. The third interlayer insulating film 949 is capable ofinsulating, and functions as a bank to separate organic compound layersof adjacent pixels from each other. This example uses a resist to formthe third interlayer insulating film 949.

The third interlayer insulating film 949 in this example has a thicknessof about 1 μm. The opening has a so-called reverse taper shape whosewidth increases as the distance from the pixel electrode 947 is closed.The reverse taper shape is obtained by coating a resist film andthereafter covering the resist film with a mask except the portion wherethe opening is to be formed, irradiating the film with UV light, andthen removing the exposed portion with a developer.

By shaping the third interlayer insulating film 949 into a reverse tapershape as in this example, organic compound layers of adjacent pixels areseparated from each other when forming the organic compound layers in alater step. Therefore cracking or peeling of organic compound layers canbe prevented even when the organic compound layers and the thirdinterlayer insulating film 949 have different coefficient of thermalexpansion.

Although a resist is used for the third interlayer insulating film inthis example, polyimide, polyamide, acrylic, BCB (benzocyclobutene), orsilicon oxide may be used instead in some cases. The third interlayerinsulating film 949 may be an organic or inorganic material as long asit is capable of insulating.

An organic compound layer 950 is formed next by evaporation. Then acathode (MgAg electrode) 951 and a protective electrode 952 are formedby evaporation. It is desirable to remove moisture completely from thepixel electrode 947 by subjecting the pixel electrode to heat treatmentprior to forming the organic compound layer 950 and the cathode 951.This example uses an MgAg electrode as the cathode of the light emittingdevice but the cathode may be formed from other known materials.

A known material can be used for the organic compound layer 950. In thisexample, the organic compound layer has a two-layer structure consistingof a hole transporting layer and a light emitting layer. The organiccompound layer may additionally have one or more layers out of a holeinjection layer, an electron injection layer, and an electrontransporting layer. Various combinations have been reported and theorganic compound layer of this example can take any of those.

The hole transporting layer of this example is formed by evaporationfrom polyphenylene vinylene. The light emitting layer of this example isformed by evaporation from polyvinyl carbazole with 30 to 40% of PBD,that is a 1,3,4-oxadiazole derivative, being molecule-dispersed. Thelight emitting layer is doped with about 1% of Coumarin 6 as greenluminescent center.

The protective electrode 952 alone can protect the organic compoundlayer 950 from moisture and oxygen, but it is more desirable to add aprotective film 953. This example uses a silicon nitride film with athickness of 300 nm as the protective film 953. The protective film andthe protective electrode 952 may be formed in succession withoutexposing the device to the air.

The protective electrode 952 also prevents degradation of the cathode951. A typical material of the protective electrode is a metal filmmainly containing aluminum. Other materials may of course be used. Sincethe organic compound layer 950 and the cathode 91 are extremely weakagainst moisture, the organic compound layer, the cathode, and theprotective electrode 952 are desirably formed in succession withoutexposing them to the air. The organic compound layer is thus protectedfrom the outside air.

The organic compound layer 950 is 10 to 400 nm in thickness (typically60 to 150 nm), and the cathode 951 is 80 to 200 nm in thickness(typically 100 to 150 nm).

Thus completed is a light emitting device structured as shown in FIG.11B. A part 954 where the pixel electrode 947, the organic compoundlayer 950, and the cathode 951 overlap corresponds to the light emittingdevice.

A p-channel TFT 960 and an n-channel TFT 961 are TFTs of the drivingcircuit and constitute a CMOS circuit. The switching TFT 962 and thedriving TFT 963 are TFTs of the pixel portion. The TFTs of the drivingcircuit and the TFTs of the pixel portion can be formed on the samesubstrate.

In the case of a light emitting apparatus using a light emitting device,its driving circuit can be operated by a power supply having a voltageof 5 to 6 V, 10 V, at most. Therefore degradation of TFTs due to hotelectron is not a serious problem. Also, smaller gate insulatingcapacitance is preferred for the TFTs since the driving circuit needs tooperate at high speed. Accordingly, in a driving circuit of a lightemitting apparatus using an light emitting device as in this example,the second impurity region 929 and the fourth impurity region 933 b ofthe semiconductor layers of the TFTs preferably do not overlap with thegate electrode 918 and the gate electrode 919, respectively.

The method of manufacturing the light emitting apparatus of the presentinvention is not limited to the one described in this example. The lightemitting apparatus of the present invention can be fabricated by a knownmethod.

In addition, Example 1 only shows the manufacturing method of a lightemitting apparatus, however, the similar laser crystallization also isapplicable to a liquid crystal display device having TFTs therein.

EFFECTS OF THE INVENTION

According to the invention, thin film transistors with controlledcharacteristics can be produced. Further, since laser beam can bescanned without moving a substrate, the throughput of crystallizationcan be increased. Further, in combination of these, high performanceliquid crystal display device and EL display device can be manufacturedat a high yield by mass production.

1. A method of manufacturing a semiconductor device comprising the stepsof: forming an amorphous semiconductor film over a substrate; scanning afirst region of the amorphous semiconductor film with a continuous wavelaser beam in a first direction to crystallize the first region whereina crystal growth proceeds in said first region along said firstdirection; changing a minor axis direction of the continuous wave laserbeam after scanning the first region; and scanning a second region ofthe amorphous semiconductor film with the continuous wave laser beam ina second direction to crystallize the second region after changing theminor axis direction wherein a crystal growth proceeds in said secondregion along said second direction, said second direction beingdifferent from the first direction.
 2. A method of manufacturing asemiconductor device according to claim 1, wherein said minor axisdirection is changed by rotating an optical system.
 3. A method ofmanufacturing a semiconductor device according to claim 2, furthercomprising a step of making a beam shape of the continuous wave laserbeam linear.
 4. A method of manufacturing a semiconductor deviceaccording to claim 1, wherein said beam shape of the continuous wavelaser beam is made linear by using at least one cylindrical lens.
 5. Amethod of manufacturing a semiconductor device according to claim 1,wherein the first region and the second region are scanned with thecontinuous wave laser beam by using a galvanomirror.
 6. A method ofmanufacturing a semiconductor device according to claim 1, furthercomprising a step of focusing the continuous wave laser beam on thefirst region and the second region by using an f-θ lens.
 7. A method ofmanufacturing a semiconductor device according to claim 2, wherein theoptical system comprises two cylindrical lenses.
 8. A method ofmanufacturing a semiconductor device comprising the steps of: forming anamorphous semiconductor film over a substrate; scanning a first regionof the amorphous semiconductor film with a continuous wave laser beam ina first direction to crystallize the first region; changing a minor axisdirection of the continuous wave laser beam by using an optical systemafter scanning the first region; scanning a second region of theamorphous semiconductor film with the continuous wave laser beam in asecond direction to crystallize the second region after changing theminor axis direction; forming a first thin film transistor using thefirst region of the semiconductor film and a second thin film transistorusing the second region of the semiconductor film, wherein said firstdirection is coincident with a charge transferring direction of thefirst thin film transistor and said second direction is coincident witha charge transferring direction of the second thin film transistor.
 9. Amethod of manufacturing a semiconductor device comprising the steps of:forming an amorphous semiconductor film over a substrate; scanning afirst region of the amorphous semiconductor film with a continuous wavelaser beam in a first direction to crystallize the first region;changing a minor axis direction of the continuous wave laser beam byusing an optical system after scanning the first region; scanning asecond region of the amorphous semiconductor film with the continuouswave laser beam in a second direction to crystallize the second regionafter changing the minor axis direction; forming a first thin filmtransistor using the first region of the semiconductor film and a secondthin film transistor using the second region of the semiconductor film,wherein a first angle between said first direction and a chargetransferring direction of the first thin film transistor and a secondangle between said second direction and a charge transferring directionof the second thin film transistor are different from each other.
 10. Amethod of manufacturing a semiconductor device comprising the steps of:forming an amorphous semiconductor film over a substrate; scanning afirst region of the amorphous semiconductor film with a continuous wavelaser beam in a first direction to crystallize the first region;changing a minor axis direction of the continuous wave laser beam byusing an optical system after scanning the first region; scanning asecond region of the amorphous semiconductor film with the continuouswave laser beam in a second direction to crystallize the second regionafter changing the minor axis direction; forming a first thin filmtransistor using the first region of the semiconductor film and a secondthin film transistor using the second region of the semiconductor film,wherein a first angle between said first direction and a chargetransferring direction of the first thin film transistor and a secondangle between said second direction and a charge transferring directionof the second thin film transistor are the same as each other.